There are a variety of types of nonvolatile memories formed over semiconductor substrates. Among these various nonvolatile memories, a flash memory storing information by accumulating electrons in a floating gate is widely used in general because of its advantage for attaining a high degree of integration.
A flash memory includes a plurality of flash memory cells over a semiconductor substrate. Each flash memory cell includes a tunnel insulating film, a floating gate, an intermediate insulating film and a control gate in this order over an active region of the semiconductor substrate.
In writing data, charge of electrons or holes is injected from the active region through the tunnel insulating film into the floating gate, thereby changing the threshold voltage of a flash memory cell. A difference is caused in the threshold voltage depending upon the presence of charge within the floating gate. The difference is allowed to correspond to data of “1” or “0”, and thus, data is written in the flash memory cell.
Since charge stored in a floating gate thus works as a carrier of information, if the charge leaks out of the floating gate in actual use, data written in the flash memory may not be correctly read. Such a failure is designated as charge loss and may be a factor in lowering the yield and the reliability of the flash memory.
One reason for the leakage of the charge is a fence-shaped conductive residue produced in forming the floating gate by patterning a conductive film. The conductive residue is produced because the intermediate insulating film works as an etching mask in the patterning and the conductive film remains beside the intermediate insulating film. Since the conductive residue is coupled to the floating gate, charge stored in the floating gate leaks out through the conductive residue. It results in encouraging the charge loss.
Japanese National Publication No. 2005-530357 discusses a process for preventing the conductive residue. In this process, a conductive spacer is additionally formed beside the conductive film and an intermediate insulating film is formed on an inclined surface of the spacer, so that the intermediate insulating film may minimally work as an etching mask in patterning the conductive film.
The process is, however, disadvantageous because the number of steps is increased by additionally forming the conductive spacer and it is understood that the cost may be increased and the yield may be lowered.